Correcting time-of-flight measurements

Abstract

In an example, a method for correcting time-of-flight measurements includes receiving a plurality of time-of-flight measurements and determining whether the time-of-flight measurements relate to a strong signal client, a weak signal client, or a preferred signal client. If the time-of-flight measurements relate to a strong signal client or a weak signal client, the method determines whether the time-of-flight measurements constitute two distinct distributions. If the time-of-flight measurements constitute two distinct distributions, a mean value for each distribution is determined. If the mean values differ by a predetermined time delay attributable to automatic gain control processing, the time-of-flight measurements corresponding to the distribution having the lower mean value are used. If the mean values differ by less than the predetermined time delay attributable to automatic gain control processing, the time-of-flight measurements corresponding to both distributions are used.

What is claimed is: 
     
       1. A method for correcting time-of-flight measurements, comprising, by a processor:
 receiving a plurality of time-of-flight measurements; 
 determining whether the time-of-flight measurements relate to a strong signal client, a weak signal client, or a preferred signal client; 
 if the time-of-flight measurements relate to a strong signal client or a weak signal client, determine whether the time-of-flight measurements constitute two distinct distributions; and 
 if the time-of-flight measurements constitute two distinct distributions,
 determine a mean value for each distribution; 
 if the mean values differ by a predetermined time delay attributable to automatic gain control processing, use the time-of-flight measurements corresponding to the distribution having the lower mean value. 
 
 
     
     
       2. The method of  claim 1 , wherein determining whether the time-of-flight measurements constitute two distinct distributions is performed using a k-means clustering technique. 
     
     
       3. The method of  claim 1 , wherein determining whether the time-of-flight measurements relate to a strong signal client, a weak signal client, or a preferred signal client comprises:
 if a mean value of signal strengths associated with packets received from the client is greater than 40 dB, determining that the time-of-flight measurements relate to a strong signal client; 
 if a mean value of signal strengths associated with packets received from the client is less than 16 dB, determining that the time-of-flight measurements relate to a weak signal client; and 
 if a mean value of signal strengths associated with packets received from the client is greater than or equal to 16 dB and is less than or equal to 40 dB, determining that the time-of-flight measurements relate to a preferred signal client. 
 
     
     
       4. The method of  claim 1 , if the mean values differ by more than or less than the predetermined time delay attributable to automatic gain control processing, discard the time-of-flight measurements. 
     
     
       5. The method of  claim 1 , wherein the processor is part of a location determination system. 
     
     
       6. The method of  claim 5 , wherein the time-of-flight measurements are measured by a wireless access point. 
     
     
       7. The method of  claim 6 , comprising:
 if the time-of-flight measurements do not constitute two distinct distributions, for each time-of-flight measurement: 
 determine an estimated distance to the client from the wireless access point using the time-of-flight measurement; 
 compare the estimated distance to an expected distance to the client from the wireless access point; and 
 use the time-of-flight measurement if the estimated distance is within a threshold from the expected distance, 
 wherein the expected distance is determined based on an estimated location of the client determined by the location determination system. 
 
     
     
       8. The method of  claim 7 , wherein the threshold is 10 meters. 
     
     
       9. The method of  claim 1 , wherein the predetermined time delay attributable to automatic gain control processing is 30 clock cycles. 
     
     
       10. A location determination system, comprising:
 a communication interface to receive from a wireless access point multiple time-of-flight measurements associated with a client; and 
 a time-of-flight correction module to:
 determine if the time-of-flight measurements need correction; and 
 if determined that the time-of-flight measurements need correction, determine which time-of-flight measurements to discard based on the use of automatic gain control by: 
 determining that the time-of-flight measurements are from two distinct distributions; and 
 selecting the time-of-flight measurements corresponding to the distribution with the lower mean value. 
 
 
     
     
       11. The location determination system of  claim 10 , wherein the time-of-flight correction module is to determine if the time-of-flight measurements need correction by determining whether the client is a strong signal client, a weak signal client, or a preferred signal client based on the mean value of the time-of-flight measurements. 
     
     
       12. The location determination system of  claim 10 , wherein using the time-of-flight measurements corresponding to the distribution with the lower mean value further comprises:
 selecting the time-of-flight measurements corresponding to the distribution with the lower mean value only if the two distributions differ by approximately a predetermined time delay attributable to automatic gain control processing. 
 
     
     
       13. The location determination system of  claim 10 , further comprising:
 a location determination module to determine a location of the client using the selected time-of-flight measurements. 
 
     
     
       14. The location determination system of  claim 13 , wherein the time-of-flight correction module is to:
 if determined that the time-of-flight measurements are not from two distinct distributions, for each time-of-flight measurement; 
 determine an estimated distance to the client from the wireless access point using the time-of-flight measurement; 
 compare the estimated distance to an expected distance to the client from the wireless access point; and 
 use the time-of-flight measurement if the estimated distance is within a threshold from the expected distance, 
 wherein the expected distance is determined based on an estimated location of the client determined by the location determination module using prior time-of-flight measurements. 
 
     
     
       15. A non-transitory computer-readable storage medium storing instructions for correcting time-of-flight that, when executed by a processor, cause the processor to:
 determine that a plurality of time-of-flight measurements correspond to a strong signal client or a weak signal client; 
 determine that the time-of-flight measurements belong to two distinct distributions; 
 determine whether a mean value of one of the distributions differs from a mean value of the other distribution by a predetermined time delay attributable to automatic gain control processing; and 
 if determined that the mean values differ by approximately the predetermined time delay attributable to automatic gain control processing, use the time-of-flight measurements corresponding to the distribution with the lower mean value to assist in estimating a location of the client.